1. Chapter 6: Cell Structure
Leaving Certificate Biology
Higher Level

2. The Microscope
A microscope is used to view very small living organisms and cells
“Be familiar with and know how to use a light microscope”

3. The Microscope
Two types of microscope you need to know for the Leaving Certificate are:
Light (compound) microscope: uses visible light, two or more lenses, and a specimen - usually stained to make structures more visible
Transmission electron microscope: uses a beams of electrons (e-), a number of electromagnetic lenses (that focus and diverge the beam of e-), a piece of photographic film (like X-ray film), and a specimen

4. The Light Microscope
Eyepiece: magnifies the image; closest to the observer’s eye
Objective: magnifies the image; closest to the specimen
Turret: holds the objective lenses
Stage: holds the specimen (slide)
Clips: holds the slide in place
Diaphragm: controls the amount of light
Light source/mirror: sends light up through the stage and specimen
Fine/coarse focus wheels: make fine/large adjustments to the clarity of the image

5. The Electron Microscope
The beam of e- from tungsten filament travels through specimen and onto photographic film
Everything in the microscope is held in a vacuum
The specimen must be coated with a heavy metal and must also be very thin as e- cannot penetrate thick specimens
Electromagnetic lenses focus the high-velocity e- beam as it hits the specimen and diverges the beam as it approaches the photographic film - thereby magnifying the image - amount of divergence dictates what the magnification will be
If the divergence of the beam is too big the magnification will be such that we would lose good resolution – thereby producing a blurry image
The maximum resolution of the electron microscope is: nanometre (nm) = mm = 1/10,000th mm

6. Using a Light Microscope
Ensure low-power lens (4X) is in position before placing specimen on stage
Separate stage and objective as much as possible using coarse wheel before placing specimen on stage
Adjust mirror or turn on light underneath stage
Ensure diaphragm is fully open to allow light pass through
Place specimen slide on stage so that specimen is directly above hole in stage
Bring 4X close to specimen - it is easier and safer to do initial focusing using low-power lens than higher-power lenses
Looking through eyepiece bring image into focus by turning the coarse and fine focus wheels towards you slowly

7. Using a Light Microscope (cont.)
Adjust diaphragm if necessary during focusing as image may be too bright to view specimen
Once in focus, move slide around gently on stage to see different fields of view
Change the objective to the 10X lens
Draw a sketch of the field of view
Change the objective to the 40X lens carefully as this lens may hit the slide and cause damage
Refocus using fine focus wheel
Once finished move the 4X lens back to the main position - only then remove slide

8. Animal Cell Structure

9. Animal Cell Structure and Function
Cell Membrane: contains the contents of the cell and controls what enters and leaves the cell
Cytoplasm: liquid of cell – consists of cytosol and cell organelles and is where chemical reactions occur
Nucleus: contains the DNA in the form of either chromosomes or chromatin and controls all of the activities of the cell
Nucleolus: where ribosomes are made
Nuclear pores: allow movement of chemical substances in and out of nucleus

10. Animal Cell Structure and Function
Nuclear envelope: double phospholipid membrane keeping the DNA separate from the cytoplasm
Chromatin: form DNA takes when the cell is not dividing - elongated chromosomes
DNA: deoxyribonucleic acid - organised into chromosomes; where genes are located
Mitochondrion: site of respiration - supplies energy for metabolism
Ribosome: site of protein synthesis

11. Plant Cell Structure

12. Plant Cell Structure and Function
Cell wall: made of cellulose; gives shape and support to plant cell; permeable to water, salts and minerals; prevents cell from bursting; protects cell from bacteria and viruses
Chloroplast: contains green pigment chlorophyll; responsible for photosynthesis
Vacuole: large, fluid-filled (sap) sac that takes up the majority the plant cell and is separated from the cytoplasm by a membrane called the tonoplast. Water, food (glucose, amino acids, etc), minerals, ions and wastes are stored here

13. 2 3 1 4 5 6 9 8 7 Nucleus Nucleolus Cell membrane Chromatin
Nuclear pore 5 6
Nuclear envelope 9
Ribosomes 8 7
Cytoplasm
Mitochondrion

14. 1 2 3 12 4 11 5 6 10 9 7 8 Nucleus Cell membrane Nucleolus Chromatin
Cell wall 11
Nuclear envelope 5
Central vacuole 6
Nuclear pore 10
Ribosome 9
Cytoplasm 7
Mitochondrion
Chloroplast 8

15. Prokaryotic and Eukaryotic Cells
Two major kinds of cells - prokaryotic and eukaryotic - can be distinguished based on their structural organisation
Prokaryotic cells have a tough outer cell wall and have no membrane-bound nucleus nor membrane-bound organelles
Eukaryotic cells have a membrane-bound nucleus and organelles

16. Mandatory Experiment: To prepare and examine one animal cell and one plant cell, unstained and stained using the light microscope at 100X and 400X

17. To prepare and examine an animal cell
Apparatus/Chemicals:
Cotton buds; microscope slides; coverslips; methylene blue; needle; tissue; water; light microscope
Method:
Scrape the inside of your mouth with cotton bud once and immediately smear on the centre of clean glass slide - do not let smear dry
Immediately add a few drops of methylene blue - leave 5 min
Blot excess stain with tissue and add a drop of water

18. To prepare and examine an animal cell
Method (continued):
Place coverslip over smear by lowering slowly from a 45º angle using the needle
Prepare another smear slide in same way but without stain
Examine both slides under microscope using correct method - at 100X and 400X
Coverslip
Glass slide
Water droplet with methylene blue
Cheek cell smear

19. To prepare and examine an animal cell
Results:
Images are visible stained and unstained cheek cells at both 100X and 400X
Based on your
experience of viewing
cells at 100X and 400X,
at what magnification
do you think this image
was obtained?
Conclusion:
Cells can be examined in detail using the light microscope
Cell numbers, structure, and organelles can be better examined by staining the cells ?X

20. To prepare and examine a plant cell
Apparatus/Chemicals:
Microscope slides; coverslips; iodine; needle; blades; tissue; water; light microscope
Method:
Using the blade carefully incise out shallow and small squares from an internal leaf of an onion
Separate a single-celled layer of tissue from one of the squares
Place this epidermal strip onto the glass slide
Add few drops of iodine - leave 5 min
Blot excess iodine with tissue and add drop of water

21. To prepare and examine a plant cell
Method (continued):
Place coverslip over epidermal strip by lowering slowly from a 45º angle using the needle
Prepare another slide in same way but without iodine
Examine both slides under microscope using correct method - at 100X and 400X
Coverslip
Glass slide
Water droplet
Onion layer
Mounted needle

22. To prepare and examine a plant cell
Results:
Images are visible of stained and unstained onion cells at both 100X and 400X
Based on your
experience of viewing
cells at 100X and 400X,
at what magnification
do you think this image
was obtained?
Conclusion:
Cells can be examined in detail using the light microscope
Cell numbers, structure, and organelles can be better examined by staining the cells ?X ?X

23. To prepare and examine an animal cell
Safety:
Care must be taken with the slides and coverslips. They are broken very easily - broken glass should be reported immediately to the teacher and/or laboratory technician.
Use a clean cotton wool buds (and not your finger) to obtain a smear of cells.
Extreme care must be taken when using the blade to obtain the epidermal strip of onion cells - a wooden chopping board should be used to minimise the risk of the onion slipping whilst cutting it
Wear latex or nitrile gloves when handling the methylene blue and iodine solutions - they easily stain the skin and their effects on humans are unknown so care must be taken (especially with substances that stain nuclei of living cells as they are potential carcinogens).
Lab coats must be worn to protect clothing